As known, electric arc furnaces used for producing molten metal from metal scrap have one or more electrodes that generate an electric arc from the electrode end to the materials placed in the furnace. The electrode is disposed vertically with respect to the furnace by mounting apparatus and extends downwardly into the furnace through a hole in the roof. The upper end of the electrode is electrically connected to a voltage source, and in operation the electrode emits an electrical arc from its oven end (or anode) to the scrap material thereby creating the energy to melt the scrap.
The electrode material is a form of graphite that is provided in solid cylindrical sections. The electrode sections range in diameter from approximately 8-inches to 30-inches and in length from approximately 5-feet to 10-feet, with weight up to approximately 4.500 pounds. Electrode sections have internally threaded ends to allow them to be screwed together using a threaded nipple with external threads. The threaded nipples are also solid graphite. As the electrode operates, the bottom of the electrode is consumed in the furnace, and periodically the furnace is shut down so that a new electrode section may be added onto the end of the electrode that protrudes from the furnace. The remaining electrode is then lowered into the operating position in the furnace that was vacated by the consumed electrode.
FIG. 1 is a figurative prior art illustration of a new electrode section 12 that is in position to be added to a fixed, mounted electrode 16. An electrode holder 14 secures the mounted electrode 16 in position so that it extends through a hole in the electric arc furnace roof 18. The electrode section 12 is vertically suspended by a crane, not shown, that is hooked onto a threaded stem lift plug 20 attached to the upper end of the electrode section 12 that is held over the fixed, mounted electrode 16. The externally threaded nipple 21 is installed in the bottom of the electrode section 12. Operators access the electrode section 12 by either standing on the electrode holder 14 or on a catwalk 22. Once the electrode sections 12, 16 are joined, the electrode holder 14 releases the fixed electrode section 16 and the crane lowers the electrode into position in the furnace. The electrode holder 14 is then tightened to hold the added electrode section 12, and the crane releases the electrode.
Among the known methods for installing of the new electrode section 12 to the electrode section 16 already installed in the furnace is to use manual tools. A manual tool commonly used is a chain wrench, which is a chain that is tightly wrapped around the free electrode body one time, with both ends of the chain fastened to a handle. The chain grasps the electrode when the handle is pulled and the free electrode is threaded into the female connector 23 of the installed electrode 16. Manual tightening of the electrode section to the specified torque is difficult.
Both mechanical integrity and electrical continuity are critical at each joint of the electrode. There is a risk of breakage of the free electrode section that can result from the attempt to reach the specified torque or from making the connection too tight, and the electrode sections are very expensive. If the connection is too loose, breakage can result from a bad connection that allows the electrodes to swing and break. Thread damage can occur from overtightening as well as from undertightening, when excessive heat builds up as a result of increased resistance and electric arcing occurs across a gap in the threads at a poor connection. Each socket of the electrode sections must be blown clean with a compressed air hose to eliminate any particles that could cause the joint to fail because of increased electrical resistance, which in turn promotes overheating, waste of power, and breakage of the connecting nipple.
The manual equipment requires from two to three people to operate it for a period ranging from twelve to fifteen minutes. Typically, an electrode section must be added for each 8 to 12-hour shift. The operators are in a less than totally safe environment when installing a new electrode section because of the difficulty involved in using the equipment on a platform in the hot and dusty area above the furnace for extended time. The extended time required for the furnace to be shut down is also undesirable.
In summary, the known manual methods for tightening the electrode sections require excessive furnace downtime and too many people to perform the operation, and it is difficult to tighten to the specified torque.